Building in Blockwork

Blockwork, or masonry build, has long dominated the homebuilding sector but it’s not often the primary choice for self-builders. So what can this traditional construction method offer to entice self-builders to choose blockwork?

It’s certainly working hard to shake off its image as a traditional walling system – with plenty of innovation and choice.

The Cavity Wall

One of the key features of contemporary masonry construction, the cavity wall, didn’t become commonplace until the 1920s. The arrival of the mass-produced concrete block made it cheaper and quicker to split the external wall into an inner and outer skin.

This method meant that the amount of relatively expensive brick or stonework required was halved. It also addressed the issue of damp — one of the main failings of the solid brick wall. The cavity in-between provides a line of defence against water penetration.

In a cavity wall, the load-bearing is taken care of by the concrete blocks used on the inner skin, while the outer skin brick or stonework just has to keep the weather at bay.

What Problems Occur with Cavity Walls?

One of the elements that causes problems in a cavity wall is the placing of the wall ties, required to stitch the inner and outer skins together structurally.

These ties can get bent out of shape, rust and collect mortar droppings, (known as ‘snots’). These dobs of mortar act as a pathway for rainwater to cross the cavity and get into the blockwork.

How to Insulate a Cavity Wall

Whereas the alternative building systems invariably set out to provide excellent insulation levels, cavity wall work struggled to keep rainwater out and heat in.

Yet over time, both the designs and the standards have improved and there are now well-worked routines for how you should build and insulate cavity walls.

This map highlighting ‘UK zones for exposure to driving rain’ is used to determine suitability for full-fill cavity wall insulation. As you can see, west-facing walls in western areas are most prone

The Building Regulations split the nation into four climate zones (above). Each zone has its own risk profile with its own prescriptions of how insulated cavity walls should be built. The move to pack more insulation into cavities has resulted in much wider cavities, which seem to be less prone to water penetration.

However, this has contributed to another issue for masonry builders — the width of the wall. On larger homes, this is not critical, but on smaller houses the area taken up by external walls can have a significant effect on room sizes.

Framed construction systems in particular can be used in conjunction with lightweight rainscreens, such as timber or tile. They can achieve the required U value in less than 300mm width where conventional masonry cavity work is struggling to do this in 400mm.

The Advantages of Blockwork

Despite all of these issues, cavity wall work remains by far the most common method throughout the UK (with the exception of Scotland which embraces timber frame).

Although cavity-based masonry work can be relatively slow, is liable to weather disruption and can be prone to error, it is also the best understood system and the most readily available.

Every builder is familiar with the cavity wall and every builders’ merchant stocks all the materials required to build one

It’s also a forgiving system: if the foundations are not quite as true as they should be, good bricklayers can sort out any issues within their first few courses

Blockwork is equally at home with extensions as it is with new build — another area which factory-built systems can struggle to cope with

Blockwork Construction Methods

Wienerberger’s Porotherm clay block is a recent innovation, with an interlocking system meaning no mortar is required on the vertical joint

Thin-Joint

This solution is not new: Tarmac Durox’s thin-joint system has been available in the UK since the 1990s.

Thin-joint block laying does away with the conventional 10mm bed of cement mortar. It replaces it with a 2-3mm glue mortar bed, similar in characteristics to tile adhesive.

It sets very rapidly and enables blocklayers to use much larger blocks, building them into walls much faster.

The larger the project, the bigger the labour savings from employing thin-joint methods. And, when used wisely, it brings with it many of the speed benefits of off-site construction methods.

Aircrete

Aircrete consists of a PFA (pulverised fuel ash) aggregate block with air bubbles, formed after a reaction of aluminium powder and lime.

The steam curing process gives the blocks a lot of strength (not as strong as concrete blocks, but still up to 9N/mm2), which means they can be used for loadbearing walls.

There are three big selling points, however:

Easier handling — they are lighter, so are quicker to work with and easier to cut

Energy performance — the air bubbles provide a good insulative performance, with thermal conductivity of up to 0.11/m2/K

As a result of the lighter weights, blocks can be manufactured in larger formats, which means quicker laying times. This is especially true when combined with a thin-joint masonry system (more on which later)

Honeycomb Clay Blockwork

This method shares some of the benefits of other thin-joint systems in that it is quick to lay and to get up to storey height in a day. But additionally it promises better insulation performance because of the honeycomb structure of the blocks.

With the backing of Wienerberger, the world’s largest brick manufacturer, Porotherm clay blocks are slowly making inroads into the conservative block market, marketed not just on speed but also on sustainability grounds.

Blockwork Innovations

The Finish

Concrete is enjoying a renaissance as a building material of choice for the self-builder, with polished concrete a popular flooring choice and the industrial style of exposed concrete finding its way onto walls. Lignacite, for example, create blocks that claim to be ‘architectural facing masonry’, which comes in all different types of colours and finishes.

Panellisation and Build Approach

The block industry is beginning to embrace this trend. After all, why build blocks one by one on a wet building site when it can be done in the factory. But, take-up is slow, mainly because builders are used to building on-site and see little benefit.

UK supplier Forterra has a factory-finished panelised system available for homebuilders.

The Future of Blockwork

The two main drivers behind the quest for new walling methods are the need to reduce labour costs on site and the need to meet higher building standards, especially as regards thermal insulation levels.

Small changes such as switching to better wall ties or more refined detailing around window and door openings may end up being more significant than entirely new methods.